A prior art method of preheating a thermoformable material, including thermoset and thermoplastic materials, involves contact heating. The contact heating method includes placing the material in direct contact with a heat source, such as thermally regulated upper and lower platens. Heat is then transferred principally by conduction from the outer surface to the inner core of the material. Because such materials are typically poor conductors of heat, however, this method requires a significant amount of time to sufficiently heat the materials.
Another method of preheating a thermoformable material involves radiant heating. This method involves placing the material near a radiant heat source, such as electric coils or ceramic heaters. The outer portions of the material, however, tend to selectively absorb the radiant energy, and core heating is again accomplished primarily by conduction. Consequently, this method also requires a significant amount of time to sufficiently heat the material.
A significant improvement over prior art methods of preheating a thermoformable material is described in commonly owned U.S. patent application Ser. No. 09/082,743, entitled "Method And Apparatus For Preheating Permeable, Thermoformable Material", which is hereby incorporated by reference in its entirety. This application provides a method and apparatus for preheating a permeable, thermoformable material having first and second sides, and includes a housing and first and second retaining members cooperable with each other to sufficiently retain the material within the housing. A fluid distribution system is disposed within the housing and is adapted to be connected to a source of heated fluid. The fluid distribution system is operative to introduce heated fluid onto the first side of the material. In addition, the housing is adapted to cooperate with a negative pressure source for developing a suction on the second side of the material to draw the heated fluid through the material, thereby convectively heating the material. In this design, the thermoformable material is held along its peripheral edges by a plurality of screws or nails which are supported on a frame. This feature requires the operator to press the thermoformable material onto the screws or nails to attach the thermoformable material to the frame, which adds labor and assembly costs to the manufacturing process, and does not lend itself to robotic handling.
Accordingly, it is desirable to provide an improved apparatus for holding the peripheral edges of the thermoformable sheet material onto a frame in a manner which is easily clamped and unclamped, and lends itself to robotic operation, and also prevents heat losses by leakage along the peripheral edges of the thermoformable sheet material.